Method of bleaching cellulose pulp and bleaching line therefore

ABSTRACT

The invention relates to a method of bleaching cellulose pulp in a bleach line having at least two bleaching steps D 1 , D 2  of alkaline or acidic type, and a bleaching line for the method, in which the filtrate distribution is led up through the bleaching line in counter-current to the flow of cellulose pulp established in the bleaching line. Instead of a conventional filtrate distribution including filtrate tanks, a single joint main conduit  1  is used for the bleaching steps of the same type. Wash filtrate obtained from the wash steps of the same type used after or before the bleaching step, is led to a branch point A 2 , A 4 , A 6  on the main conduit  1 , that is positioned after the branch point for drawing off of wash and/or dilution liquor for the wash step in question A 1 , A 3 , A 5 , A 7 . All branch points in the joint main conduit are in open communication with each other in the main conduit, at least as seen in a direction from the last bleaching step. Hereby, a simplified and improved filtrate distribution system with an improved runnability is obtained.

[0001] The present invention relates to a method of bleaching cellulosepulp and a bleach line for the method.

PRIOR ART

[0002] It is a desire in multi-stage bleaching of cellulose pulp toreduce the water requirement and the quantity of outlet contaminatedprocess water, which contaminated process water is either pumped tosewage, possibly via sedimentation basins and/or to costlydestruction/deposition. With the object of reducing the liquorquantities, the bleaching department is ever more closed and most oftenthe process water is lead in counter-current to the direction of flow ofthe cellulose pulp in the process steps of the bleaching line.Accordingly, the fresh water or the clean process water is used in thewash of the last step and the wash filtrate obtained there from is ledas wash liquor to the wash of the preceding step and further up throughthe bleaching line. Commonly, a number of filtrate tanks are usedbetween each bleaching step wash for this leading of wash filtratethrough the process, with the object of guaranteeing supply of washliquor and securing that the wash liquor is lead in counter-current tothe flow of cellulose pulp. In addition to such filtrate tanks,expensive control and regulation systems with valves are required forthis handling of the wash liquor in filtrate tanks, to monitor thelevels in the filtrate tanks since the risk can not be taken that afiltrate tank is emptied whereby it may cause a stoppage in the wash ofthe bleaching step in question. The number or filtrate tanks alsoresults in a risk of an increased outlet of odorous gases as allfiltrate tanks require ventilation in order to level out changes in thevolume in the filtrate tanks. Often, special degassing systems arerequired to handle and destruct such odorous gases. Accordingly, bigadvantages could be attained if the number of filtrate tanks between theprocess steps could be minimised.

THE OBJECT AND PURPOSE OF THE INVENTION

[0003] One object of the invention is to reduce the need of, and in somecases completely eliminate, such expensive filtrate tanks, controlsystems and valves in the wash liquor systems, whereby the investmentcosts for the bleaching line may be strongly reduced. The reduced numberof necessary filtrate tanks also results in the possibility of a morecompact and more optimal design of the bleaching line, withoutconsideration of such filtrate tanks that conventionally numbers to atleast the same number as the number of bleaching steps, with a moreefficient layout of the bleaching steps of the bleaching line.

[0004] Yet another object is to increase the runnability/accessabilityof the system as several control valves may be removed, that otherwiseare always potential risks for plugging/stoppage of the liquordistribution system.

[0005] Yet another object is to improve the runnability as the risk ofmixing-in of air in the filtrate system is considerably reduced when thenumber of filtrate tanks can be considerably reduced. At the same time,accumulation of floating pulp is avoided, which floating pulp usuallyaccumulates after a certain running time, by surface flotation infiltrate tanks. Principally in alkaline steps, such accumulated floatingpulp may rise to a level of a few metres above the surface in thefiltrate tanks and it must be continuously taken care of or recycled tothe bleaching line in order not to risk plugging of the filtratesystems.

[0006] Yet another object is that the bleaching plant can be renderedmore environmentally friendly as occasional overloads in certainpositions, so called over-runs, need not result in outlet of gas orliquor. Yet another object is to minimise the water consumption. By thesystem, the system itself may compensate for occasional changes in washliquor requirements in the various bleaching steps and secure that arequired wash liquor quantity is always guaranteed the bleaching steps.

[0007] It is yet another object to minimise the energy consumption inpumps in the filtrate distribution system, where instead a pressurisedfiltrate main conduit is maintained and any required liquor quantity isdrawn off form the main conduit, as needed.

[0008] Yet another object is to decrease the length of the tube system,which reduces the costs of installation and the complexity of thesystem, whereby in the latter case the lucidity is also increased forthe operators.

[0009] A cost reduction of between 1 and 2 millions USD can be obtainedfor a 4-step bleaching line D₀-EOP-D₁-D₂ with intermediate wash steps,if the invention is fully applied.

LIST OF DRAWINGS

[0010]FIG. 1 shows a conventional bleaching sequence D₀-EOP-D₁-D₂ inwhich the filtrate is led in counter-current, via filtrate tanks;

[0011]FIG. 2 shows the same bleaching sequence D₀-EOP-D₁-D₂ in which thefiltrate is lead between the steps in accordance with the invention.

PRIOR ART

[0012]FIG. 1 shows a conventional bleaching sequence D₀-EOP-D₁-D₂ inwhich the filtrate from the washing between the steps is led incounter-current between the bleaching steps, via filtrate tanks FT₁-FT₄.The pulp is pumped by a pump from a first storage tower, to a first washW₁ in which the pulp is washed with a clean first filtrate FF1. In thefigure, wash apparatuses of wash press type are schematically shown,having two contra-rotating wash drums where wash liquor is supplied tothe web of pulp on both drums, but subsequently the figure only showsthe supply of wash liquor to one drum of the wash press. It is typicalto such bleaching systems having wash presses that the bleaching takesplace in reactors at a pulp consistency of 10-14% and that aftertreatment in the reactor, the pulp is diluted to about 5-10%, typicallyabout 8%, before it is fed to the wash press. After the wash press, thepulp has a consistency of 20-35%, typically 30%.

[0013] After the first wash W₁, the washed pulp is fed down into a chutein which the pulp is diluted by a liquor that is pumped from a filtratetank FT₁, from which chute it is pumped by a pump and a subsequent mixerM1 to a subsequent bleaching step, here a first chlorine dioxide step D₀shown as an up-flow tower (the pulp flows upwards in the tower). Thechemicals for the bleaching step, ClO₂ and acidifier H₂SO₄, are mixed-inby the mixer M1 before the pulp is led to the D₀ bleaching tower. Afterthe bleaching in the D₀ bleaching tower, the pulp is led to a chute inwhich the pulp is diluted by filtrate from the first filtrate tank FT₁.From the chute, the pulp is pumped to a subsequent wash W₂ in which thepulp is washed by wash liquor from a third filtrate tank FT₃.Thereafter, the pulp washed in the wash W₂ is led to a chute in whichthe pulp is diluted by filtrate from a second filtrate tank FT₂, andfrom the chute the pulp is pumped by a pump and a subsequent mixer M2 toa subsequent alkaline extraction step, here an EOP step shown as anup-flow tower. The chemicals for the extraction step, NaOH and peroxideH₂O₂, and oxygen gas if needed, are mixed in by the mixer M2 before thepulp is led to the EOP tower. After the treatment in the extractiontower EOP, the pulp is led to a chute in which the pulp is diluted byfiltrate from the second filtrate tank FT₂, where after the pulp ispumped to a subsequent wash W₃. In the wash W₃, the pulp is washed byclean filtrate FF2. After the wash W₃, the washed pulp is fed down to achute in which the pulp is diluted by filtrate from a third filtratetank FT₃, where after the pulp is pumped by a pump and a subsequentmixer to a subsequent bleaching step, here a second chlorine dioxidestep D₁, shown as an up-flow tower. The chemicals for the D₁ bleachingstep, ClO₂ and pH-adjuster, are mixed in by the mixer before the pulp isled to the D₁ bleaching tower. As an alternative, the adjusting of pHcan take place by addition of e.g. NaOH in the preceding chute. Afterthe treatment in the D₁ bleaching tower, the pulp is led to a chute inwhich the pulp is diluted by filtrate from the third filtrate tank FT₃,where after the pulp is pumped to a subsequent wash W₄. In the wash W₄,the pulp is washed by filtrate from a fourth filtrate tank FT₄.

[0014] After the wash W₄, the washed pulp is fed down to a chute inwhich the pulp is diluted by filtrate from a fourth filtrate tank FT₄,where after the pulp is pumped by a pump and a subsequent mixer to asubsequent bleaching step, here a third chlorine dioxide step D₂, shownas an up-flow tower. The chemicals for the D₂ bleaching step, ClO₂ andpH-adjuster, are mixed in by the mixer before the pulp is led to the D₂bleaching tower. As an alternative, the adjusting of pH can take placeby addition of e.g. NaOH in the preceding chute. After the treatment inthe D₂ bleaching tower, the pulp is led to a chute in which the pulp isdiluted by filtrate from the fourth filtrate tank FT₄, where after thepulp is pumped to a subsequent wash W₅. In the wash W₅, the pulp iswashed by clean filtrate FF3. The pulp bleached by the shown bleachingsequence, D₀-EOP-D₁-D₂, is subsequently led to a storage tower (notshown) and typically has a brightness above ISO 80 and is often a fullybleached pulp of ISO 90. In certain cases, a subsequent treatment can beused to modify the properties of the pulp in respect of drainageproperties etc. The main principle of the filtrate distribution of theshown bleaching sequence is that there are filtrate tanks between thetreatment steps, which filtrate tanks receive the filtrate from the washin question. The filtrate tank FT₄ of the last wash W₅ collects thefiltrate and then the filtrate is led in counter-current to the flow ofpulp through the bleaching line, via pumps, and is used as dilution orwash liquor in preceding positions. In a corresponding manner, filtratefrom the wash apparatuses W₄, W₃, W₂ is collected in the filtrate tanksFT₃, FT₂ and FT₁, respectively, and then the filtrate is led via pumps,from the respective tank in counter-current to the flow of pulp throughthe bleaching line. In certain circulations, a certain share of thefiltrate is also bled off, as is shown in the feed from the filtratetanks FT₁ and FT₂, in order to avoid accumulation of increasing contentsof undesired substances, which bleeding-off is compensated by supply ofcleaner filtrates FF1 and FF2. The bleeding-off of filtrate is theprinciple outlet from the bleaching line. In this counter-currentfiltrate distribution, alkaline filtrate is separated from acidic.Accordingly, the alkaline filtrate from the EOP step is collected in thefiltrate tank FT₂, and no acidic filtrate is used in the wash W₃, butinstead clean filtrate FF2 is used. In certain applications, suchalkaline filtrate can be fed on, upstream, to the oxygendelignification, where it is used as wash liquor in the wash after theoxygen delignification. For the acidic filtrates that are collected inthe filtrate tanks FT₄, FT₃ and FT₁, the wash liquor is led strictlycounter-current to the flow of pulp, i.e. from FT₄ to FT₃, and finallyto FT₁ from where the acidic filtrate is bled off from the bleachingdepartment since it can not be handled in the recovery system, mainlydue to high contents of chloride that destroy the soda recovery boiler.

PREFERRED EMBODIMENT OF THE INVENTION

[0015]FIG. 2 shows an embodiment of the invention, in which the filtratedistribution system instead of the large number of filtrate tanks, hasbeen replaced by a joint main conduit 1 for all acidic bleaching steps.

[0016] Here, bleaching of the cellulose pulp takes place in a bleachingline with at least two bleaching steps in the bleaching line and at somepoint including a first and a second bleaching step D₁, D₂ insuccession, as seen in the flow direction of the cellulose pulp, whichbleaching steps have wash apparatuses W₄ and W₅ for the pulp arrangedafter the first and the second bleaching step, respectively. Wash liquorand where appropriate dilution liquor, is led in principle incounter-current to the flow of pulp through the bleaching steps of thebleaching line, which flow of pulp (bold arrows are flow lines) passesthrough the sequence W₁-D₀-W₂-EOP-W₃-D₁-W₄-D₂-W₅. The wash liquor issupplied to a main conduit 1 that is arranged in parallel to thebleaching line, by a pump P20 from a filtrate tank FT₂, and at least oneof wash liquor and dilution liquor is taken to the subsequent wash W₅ ofthe second bleaching step D₂, from a first branch position A1 in themain conduit and at least a part of the wash filtrate from thesubsequent wash of the second bleaching step is led to a second branchposition A2 in the main conduit. At least one liquor of wash liquor anddilution liquor is taken to the subsequent wash W₄ of the firstbleaching step D₁, from a third branch position A3 in the main conduit,and at least a part of the wash filtrate from the subsequent wash of thefirst bleaching step is led to a fourth branch position A4 in the mainconduit. Here, the branch positions A1-A4 connect to the main conduitwith the first branch position A1 arranged first, as seen in thedirection of flow in the main conduit 1, and the second to fourth branchpositions A2-A4 in succession thereafter, an open communication beingestablished in the main conduit between the branch positions A1-A4.Upstream said first branch position A1 at the first end of the mainconduit, a main pump device P20 is arranged which pressurises the mainconduit and establishes a basic flow in the main conduit in a directionreverse to the formed flow of cellulose pulp in the bleaching line. Inthis embodiment, wash liquor is led from the first and the third branchposition A1 and A3, respectively, to the respective wash apparatus W₅and W₄, respectively, via pump devices P21 and P22, respectively.

[0017] The inventive main conduit may suitably be used for additionalbleaching steps of the same type, if the main conduit, as in the figure,is used for number of acidic steps or a number of alkaline steps (thelatter not shown in FIG. 2).

[0018] Accordingly, at least one additional bleaching step D₀ may beprovided before the first and second bleaching steps D_(1 and D) ₂,respectively, as seen in the flow direction of the cellulose pulp, afterwhich additional bleaching step D₀ there is a wash apparatus W₂ for thepulp. At least one liquor of wash liquor and dilution liquor (washliquor in the figure) is taken to the subsequent wash W₂ of theadditional bleaching step, from a fifth branch position A5 in the mainconduit 1 and at least a part of the wash filtrate from the subsequentwash W₂ of the additional bleaching step is led to a sixth branchposition A6 in the main conduit. The branch positions connect to themain conduit with the fifth branch position A5 arranged after the fourthbranch position A4, as seen in the direction of flow in the main conduit1, and the sixth branch position A6 in succession thereafter, an opencommunication being established in the main conduit between the branchpositions A1-A6.

[0019] In FIG. 2, an alkaline extraction step EOP or alternatively an EOstep without peroxide charge is, in a per se conventional manner,arranged after the additional bleaching step D₀ and before the firstbleaching step D₁, as seen in the direction of flow of the cellulosepulp through the bleaching line, and a wash apparatus W₃ is arrangedafter the extraction step EOP. The wash filtrate from the subsequentwash W₃ of the extraction step can be collected in a filtrate tank FT₁and is suitably used as dilution liquor before the extraction step and apart of the wash filtrate can if needed be drawn off from the bleachingline, to sewage 11, or be led forward to an oxygen delignification step.In this embodiment, the cellulose pulp is washed in a wash apparatus W₁before the additional bleaching step D₀, as seen in the direction offlow of the cellulose pulp through the bleaching line and at least oneliquor of wash liquor and dilution liquor is taken to this washapparatus W₁, in the figure only dilution liquor, from a seventh branchposition A7 in the main conduit.

[0020] According to the shown embodiment, at least chlorine dioxide, orsome other bleaching chemical that is compatible throughout thebleaching steps, is used as active bleaching agent in the bleachingsteps D₀, D₁ and D₂, which chlorine dioxide is added to the pulp beforethe respective bleaching step in a blending apparatus M1, M3 and M4,respectively.

[0021] At the other end of the main conduit, as seen after the branchpoints A1-A7, an outlet 10 is suitably provided, from which wash liquorand filtrate can be drawn off. Preferably, the outlet is controlled by aflow controlling control valve FC, which control valve can establish acertain basic flow and/or a desired bleed-off level of filtrate, duringnormal operation. The pump P20 is controlled by a pressure regulator PC,enabling feed-back control of the main pump device P20 in order tosecure a predetermined pressure and/or flow throughout the entire mainconduit 1. Suitably, the flow controlling valve can establish a desiredflow to the outlet 10 as long as the pressure in the main conduit can bemaintained. In an alternative embodiment, the flow controlling valve FCmay be a fixed or variable throttle valve with a high pressure drop overthe valve.

[0022] By the embodiment shown in FIG. 2, a bleaching line is providedfor the bleaching of cellulose pulp, having at least two bleaching stepscomprising a first and a second bleaching step D₁ and D₂, respectively,as seen in the flow direction of the cellulose pulp, which bleachingsteps have wash apparatuses W₄ and W₅ for the pulp arranged after thefirst and the second bleaching step, respectively, and in which washliquor and where appropriate dilution liquor is led in principle incounter-current to the pulp flow through the wash apparatuses W₁-W₅ inthe bleach line W₁-D₀-W₂-EOP-W₃-D₁-W₄-D₂-W₅. The wash liquor is suppliedin a main conduit 1 that is arranged in parallel to the bleaching line,and at least one of wash liquor and dilution liquor is taken to asubsequent wash W₅ of a second bleaching step D₂, from a first branchposition A1 in the main conduit 1 and at least a part of the washfiltrate from the subsequent wash of the second bleaching step is led toa second branch position A2 in the main conduit. At least one liquor ofwash liquor and dilution liquor is taken to a subsequent wash W₄ of afirst bleaching step D₁ from a third branch position A3 in the mainconduit 1, and at least a part of the wash filtrate from the subsequentwash of the first bleaching step is led to a fourth branch position A4in the main conduit. According to the invention, the branch positionsA1-A4 connect to the main conduit 1 with a first branch position A1arranged first, as seen in the direction of flow in the main conduit,and second A2 to fourth A4 branch positions in succession thereafter, anopen communication being established in the main conduit between thebranch positions A1-A4.

[0023] Pressurisation and distribution of filtrate takes place via amain pump device P20, arranged upstream the first branch position A1 inthe main conduit. The low side of the main pump device P20 is connectedto a liquor tank FT₂, and its high side pressurises the main conduit 1and establishes a basic flow in the main conduit, reverse to the formedflow of cellulose pulp in the bleaching line.

[0024] From the first and third branch positions A1 and A3,respectively, wash liquor is led from the main conduit 1 to therespective wash apparatuses W₅ and W₄, via pump devices P21 and P22,respectively, and associated tubing.

[0025] The bleaching line preferably also includes at least oneadditional bleaching step D₀, which is arranged before the first andsecond bleaching steps D₁ and D₂, as seen in the direction of flow ofthe cellulose pulp. After this additional bleaching step D₀, a washapparatus W₂ for the pulp is arranged. At least one liquor of washliquor and dilution liquor is taken to the subsequent wash W₂ of theadditional bleaching step, from a fifth branch position A5 in the mainconduit 1 and at least a part of the wash filtrate from the subsequentwash of the additional bleaching step is led to a sixth branch positionA6 in the main conduit 1. The branch positions A5-A6 connect to the mainconduit 1 with the fifth branch position A5 arranged after the fourthbranch position A4, as seen in the direction of flow in the mainconduit, and the sixth branch position A6 in succession thereafter, anopen communication being established in the main conduit between thebranch positions A1-A6.

[0026] An extraction step is arranged in the shown bleaching line,preferably of EOP or EO type, which is arranged after the additionalbleaching step D₀ and before the first bleaching step D₁, as seen in thedirection of flow of the cellulose pulp through the bleaching line, anda wash apparatus W₃ is arranged after the extraction step. The washfiltrate from the subsequent wash W₃ of the extraction step is led to afiltrate tank FT₁, via a conduit, and filtrate from the filtrate tankis, at least partly, led as dilution liquor after the wash step W₂subsequent to the additional bleaching step D₀, via pump P30 andconduits, and a part of this wash filtrate is when needed drawn off fromthe process, preferably via an outlet from the filtrate tank FT₁. Asshown in the figure, a part of the liquor in the filtrate tank may alsobe used as dilution liquor in the chute after the EOP reactor.

[0027] In the bleaching line, cellulose pulp is washed in a washapparatus W₁ before the additional bleaching step D₀, as seen in thedirection of flow of the cellulose pulp through the bleaching line, andto this wash apparatus W₁ at least one liquor of wash liquor anddilution liquor is led from a seventh branch position A7 in the mainconduit 1 to the wash apparatus W₁, via a pump device P24 and associatedtubing. If the pulp in the storage tower ST is acidic, both wash anddilution liquor in and after the wash W₁, respectively, can be takenfrom the main conduit. But if the pulp in the storage tower is alkaline,a cleaner alkaline filtrate or a clean filtrate is used as wash liquorin the wash apparatus W₁, where the use of a clean filtrate is shown inFIG. 2.

[0028] In the bleaching steps D₀, D₁, D₂ of the bleaching line, at leastchlorine dioxide is charged as active bleaching agent or some otherbleaching chemical that is compatible throughout the bleaching steps,such as a chelating agent, a pH adjuster or some additional bleachingchemical, which chlorine dioxide or bleaching chemical is added to thepulp before the respective bleaching step in a blending apparatus M1, M3and M4, respectively.

[0029] At the end of the main conduit 1, as seen after the branch pointsA1-A7, an outlet 10 is provided, from which wash liquor and filtrate canbe drawn off from the main conduit. Suitably, the outlet 10 iscontrolled as is described above, by a pressure and/or flow controllingcontrol valve PC and/or FC.

[0030] The invention can be varied in a number of ways, within the scopeof the claims. The bleaching steps that in their subsequent washapparatuses have a joint main conduit that receives wash filtrate anddilution and/or wash liquor may, for example, all be of alkaline type orthe bleaching chemicals in question may be compatible/blendable. Inmulti-stage bleaching sequences, a main conduit may be used for thealkaline filtrate from two or more alkaline steps and another mainconduit may be used for the acidic filtrate from two or more acidicsteps.

[0031] In the embodiment shown in FIG. 2, the pumps P21-P24 are placedin the feed conduits from the main conduit. In an alternativeembodiment, powerful pumps may be provided in the return conduits thatconnect to the branch points A2, A4 and A6, respectively, which in suchcase, together with the main pump P20, pressurise the entire mainconduit. With a pressure in the main conduit established at 4 bar, pumpsin the feed conduits for dilution and/or wash liquor can normally beeliminated. The supply of dilution liquor after wash normally requires avery low pressure of about 1 bar, why a throttle is required for suchdilution liquor supply. Normally, the dilution liquor is supplied to anatmospheric dilution screw in which fluffed-up pulp of high consistency,about 30%, is blended with dilution liquor to a consistency suitable forsubsequent pumping. Therefore, there is a low pressure need on theliquor supply. In another, alternative embodiment, a basic pressure ofabout 1 bar may be established in the main conduit, which is enough tofeed dilution water, but in which a supplying pump is provided in thefeed conduit for the wash liquor. Normally, wash liquor is added in aconverging wash slot in a wash press at a higher pressure and normally,a wash liquor pressure of at least 2-4 bar is required in this position.As an additional precautionary measure, a check valve may be providedbetween the branch positions for filtrate recycling to the main conduitand feeding of dilution and/or wash liquor to the wash apparatus inquestion, especially if the branch positions of construction reasons areclose to each other. The most important aspect is that an opencommunication is established between all branch points in the mainconduit, as seen in the direction from the first end of the mainconduit, with the filtrate tank FT2, to the second end of the mainconduit, with the outlet 10.

[0032] Other wash apparatuses than wash presses may of course be used.At less heavy requirements on chemical carry-over to the subsequentbleaching step, ordinary filters or simple presses (without washing) mayof course be used, in which the filtrate from the filter or the simplepress is led to the main conduit and optional dilution liquor before thefilter or the press is taken from the main conduit. Also, wash pressessuch as a filter or a simple press without wash, may be connected to ajoint main conduit.

1. A method of bleaching cellulose pulp in a bleaching line, having atleast two bleaching steps comprising a first (D₁) and a second (D₂)bleaching step, as seen in the direction of flow of the cellulose pulp,which bleaching steps have wash apparatuses (W₄, W₅) for the pulparranged after the first and the second bleaching step, respectively,and in which wash liquor and where appropriate dilution liquor is led inprinciple in counter-current to the pulp flow through the bleachingsteps in the bleach line (W₁-D₀-W₂-EO/EOP-W₃-D₁-W₄-D₂-W₅), characterisedin that the wash liquor is supplied in a main conduit (1) that isarranged in parallel to the bleaching line, that at least one of washliquor and dilution liquor is taken to the subsequent wash (W₅) of thesecond bleaching step (D₂), from a first branch position (A1) in themain conduit and at least a part of the wash filtrate from thesubsequent wash of the second bleaching step is led to a second branchposition (A2) in the main conduit, that at least one liquor of washliquor and dilution liquor is taken to the subsequent wash (W₄) of thefirst bleaching step (D₁) from a third branch position (A3) in the mainconduit, and at least a part of the wash filtrate from the subsequentwash of the first bleaching step is led to a fourth branch position (A4)in the main conduit, in which the branch positions (A1-A4) connect tothe main conduit with the first branch position (A1) arranged first, asseen in the direction of flow in the main conduit, and the second tofourth branch positions (A2-A4) in succession thereafter, an opencommunication being established in the main conduit between the branchpositions (A1-A4).
 2. A method according to claim 1, characterised inthat the main conduit is connected to receive and distribute acidicfiltrate from and to, respectively, acidic bleaching steps.
 3. A methodaccording to claim 1, characterised in that the main conduit isconnected to receive and distribute alkaline filtrate from and to,respectively, alkaline bleaching steps.
 4. A method according to claim 2or 3, characterised in that upstream said first branch position (A1) inthe first end of the main conduit, a main pump device (P20) is providedwhich pressurises the main conduit and establishes a basic flow in themain conduit in a direction reverse to the formed flow of cellulose pulpin the bleaching line.
 5. A method according to claim 4, characterisedin that after the first (A1) and third (A3) branch positions, washliquor is led to the respective wash apparatuses (W₅, W₄), via pumpdevices (P21, P22).
 6. A method according to claim 4 or 5, characterisedin that before the second (A2) and fourth (A4) branch positions,filtrate is led to the main conduit (1), via pump devices (P21′, P22′).7. A method according to claim 1, 5 or 6, characterised in that at leastone additional bleaching step (D₀) is provided before the first andsecond bleaching steps, as seen in the direction of flow of thecellulose pulp, after which additional bleaching step a wash apparatus(W₂) is provided for the pulp, that at least one liquor of wash liquorand dilution liquor is taken to the subsequent wash of the additionalbleaching step, from a fifth branch position (A5) in the main conduit(1) and that at least a part of the wash filtrate from the subsequentwash of the additional bleaching step is led to a sixth branch position(A6) in the main conduit, in which the branch positions connect to themain conduit with the fifth branch position arranged after the fourthbranch position, as seen in the direction of flow in the main conduit,and the sixth branch position in succession thereafter, an opencommunication being established in the main conduit between the branchpositions.
 8. A method according to claim 7, characterised in that anextraction step (EO/EOP) is provided after the additional bleaching stepand before the first bleaching step, as seen in the direction of flow ofthe cellulose pulp through the bleaching line, and that a wash apparatus(W₃) is arranged after the extraction step.
 9. A method according toclaim 8, characterised in that the wash filtrate from the subsequentwash of the extraction step, at least partly is used as dilution liquorfor the wash step subsequent to the additional bleaching step, and thata part of this wash filtrate when needed is drawn off from the process.10. A method according to claim 7, characterised in that the cellulosepulp is washed in a wash apparatus before the additional bleaching step,as seen in the direction of flow of the cellulose pulp through thebleaching line, and that at least one liquor of wash liquor and dilutionliquor is taken to this wash apparatus from a seventh branch position inthe main conduit.
 11. A method according to any one of the precedingclaims, characterised in that at least chlorine dioxide, or some otherbleaching chemical that is compatible throughout the bleaching steps, isused as active bleaching agent in the bleaching steps, which chlorinedioxide is added to the pulp in a blending apparatus before thebleaching step.
 12. A method according to any one of the precedingclaims, characterised in that at the other end of the main conduit, asseen after the branch points (A1-A7), an outlet (10) is provided, fromwhich wash liquor and filtrate can be drawn off.
 13. A method accordingto claim 12, characterised in that the outlet is controlled by apressure and/or flow controlling control valve, which control valve canachieve feed-back control of the main pump device to secure apredetermined pressure and/or flow throughout the entire main conduit(1).
 14. A bleaching line for bleaching of cellulose pulp, having atleast two bleaching steps comprising a first (D₁) and a second (D₂)bleaching step, as seen in the flow direction of the cellulose pulp,which bleaching steps have wash apparatuses (W₄, W₅) for the pulparranged after the first and the second bleaching step, respectively,and in which wash liquor and where appropriate dilution liquor is led inprinciple in counter-current to the pulp flow through the washapparatuses (W₁-W₅) of the bleaching steps in the bleach line(W₁-D₀-W₂-EOP-W₃-D₁-W₄-D₂-W₅) characterised in that the wash liquor issupplied in a main conduit (1) that is arranged in parallel to thebleaching line, that at least one of wash liquor and dilution liquor istaken to the subsequent wash (W₅) of the second bleaching step (D₂),from a first branch position (A1) in the main conduit (1) and at least apart of the wash filtrate from the subsequent wash of the secondbleaching step is led to a second branch position (A2) in the mainconduit, that at least one liquor of wash liquor and dilution liquor istaken to the subsequent wash (W₄) of the first bleaching step (D₁) froma third branch position (A3) in the main conduit (1), and at least apart of the wash filtrate from the subsequent wash of the firstbleaching step is led to a fourth branch position (A4) in the mainconduit, in which the branch positions (A1-A4) connect to the mainconduit (1) with the first branch position (A1) arranged first, as seenin the direction of flow in the main conduit, and the second (A2) tofourth (A4) branch positions in succession thereafter, an opencommunication being established in the main conduit between the branchpositions (A1-A4).
 15. A bleaching line according to claim 14,characterised in that the main conduit is connected to receive anddistribute acidic filtrate from and to acidic bleaching steps,respectively, via conduits that receive acidic wash filtrate from washapparatuses and via conduits that distribute wash apparatuses withacidic wash filtrate, respectively.
 16. A bleaching line according toclaim 14, characterised in that the main conduit is connected to receiveand distribute alkaline filtrate from and to alkaline bleaching steps,respectively, via conduits that receive alkaline wash filtrate from washapparatuses and via conduits that distribute wash apparatuses withalkaline wash filtrate, respectively.
 17. A bleaching line according toclaim 15 or 16, characterised in that a main pump device (P20) isarranged upstream the first branch position (A1) in the main conduit,which on its low side is connected to a liquor tank (FT₂) and the highside of which pressurises the main conduit (1) and establishes a basicflow in the main conduit, reverse to the formed flow of cellulose pulpin the bleaching line.
 18. A bleaching line according to claim 17,characterised in that from the first (A1) and third (A3) branchpositions, wash liquor is led to the respective wash apparatuses (W₅,W₄), via respective pump devices (P21, P22) and associated tubing.
 19. Ableaching line according to claim 17 or 18, characterised in that beforethe second (A2) and fourth (A4) branch positions, filtrate is led to themain conduit (1), via pump devices (P21′, P22′).
 20. A bleaching lineaccording to claim 14, 17 or 19, characterised in that at least oneadditional bleaching step (D₀) is provided before the first (D₁) andsecond (D₂) bleaching steps, as seen in the direction of flow of thecellulose pulp, after which additional bleaching step a wash apparatus(W₂) is provided for the pulp, that at least one liquor of wash liquorand dilution liquor is taken to the subsequent wash (W₂) of theadditional bleaching step, from a fifth branch position (A5) in the mainconduit (1) and that at least a part of the wash filtrate from thesubsequent wash of the additional bleaching step is led to a sixthbranch position (A6) in the main conduit (1), in which the branchpositions (A5-A6) connect to the main conduit (1) with the fifth branchposition (A5) arranged after the fourth branch position (A4), as seen inthe direction of flow in the main conduit, and the sixth branch position(A6) in succession thereafter, an open communication being establishedin the main conduit between the branch positions (A1-A6).
 21. Ableaching line according to any one of the preceding claims 14, 15 or17-20, characterised in that an extraction step (EOP) is provided afterthe additional bleaching step (D₀) and before the first bleaching step(D₁), as seen in the direction of flow of the cellulose pulp through thebleaching line, and that a wash apparatus (W₃) is arranged after theextraction step.
 22. A bleaching line according to claim 21,characterised in that the wash filtrate from the subsequent wash (W₃) ofthe extraction step is led to a filtrate tank (FT₁), via a conduit, andfiltrate from the filtrate tank is, at least partly, led as dilutionliquor to the wash step (W₂) subsequent to the additional bleaching step(D₀), via pump (P30) and conduits, and a part of this wash filtrate iswhen needed drawn off from the process, preferably via an outlet fromthe filtrate tank (FT₁).
 23. A bleaching line according to claim 19,characterised in that cellulose pulp is washed in a wash apparatus (W₁)before the additional bleaching step (D₀), as seen in the direction offlow of the cellulose pulp through the bleaching line, and to this washapparatus (W₁) at least one liquor of wash liquor and dilution liquor isled from a seventh branch position (A7) in the main conduit (1) to thewash apparatus (W₁), via a pump device (P24) and associated tubing. 24.A bleaching line according to any one of the preceding claims 11-12 or14-19, characterised in that at least chlorine dioxide, or some otherbleaching chemical that is compatible throughout the bleaching steps, ischarged as active bleaching agent in the bleaching steps (D₀, D₁, D₂),which chlorine dioxide or bleaching chemical is added to the pulp in ablending apparatus (M1, M3, M4) before the respective bleaching step.25. A bleaching line according to any one of the preceding claims 11-20,characterised in that at the end of the main conduit (1), as seen afterthe branch points (A1-A7), an outlet (10) is provided, from which washliquor and filtrate can be drawn off from the main conduit.
 26. Ableaching line according to claim 21, characterised in that the outlet(10) is controlled by a pressure or flow controlling control valve(PC/FC), which control valve, via a feed-back device, has a feed-backcoupling to the main pump device (P20) for controlling of the main pumpdevice, to secure a predetermined pressure and/or flow through theentire main conduit (1).